Crossed guide microwave coupler



6 7,1970 UBRUMBAUGH: CROSSED GUIDE mmwimwmm Filed March 5, 1968 FIG. I

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Oct. 27, 1970 c. r. BRUMBAUGH" CROSSED GUIDE mcnovuwa .cou'rwa Filed Mafch .5, 1968 FIG. 20

.250 DIA. POST m TOP cums INPUT PORT .250 DIA. PosT" IN BOTTOM. GUIDE/"'- FIG. 4v

CROSSED GUIDE MICROWAVE COUPLER Filed Mar ch 5.; 1 968 -i$ Sheets-Sheet :5

FIG. .28

IN VEN TOR.

CHARLES T. :BRUMBAUGH Anonmzv United States Patent Oflice 3,537,037 CROSSED GUIDE MICROWAVE COUPLER Charles T. Brumbaugh, Norwalk, Calif., assignor to North American Rockwell Corporation Filed Mar. 5, 1968, Ser. No. 710,559 Int. Cl. H01p 5/12 U.S. Cl. 333-11 4 Claims ABSTRACT OF THE DISCLOSURE A wideband microwave directional coupler for a crossed pair of rectangular microwave waveguides. A common broadwall of the crossed guide pair is apertured, a first and second mutually spaced slotted aperture of the common wall being respectively parallel to a respective one of the two diagonals of the common wall.

CROSS-REFERENCES TO RELATED APPLICATIONS U.S. application Ser. No. 621,001 filed Mar. 6, 1967, by I. A. Algeo, et al. for Frequency-sensitive Cross-Scanning Antenna.

U.S. application Ser. No. 649,166 filed June 27, 1967, by J. A. Algeo for A Crossed Guide Directional Coupler.

U.S. application Ser. No. 699,756 filed Jan. 15, 1968, by C. A. Wiley for Multi-Mode Antenna.

BACKGROUND OF THE INVENTION The utilization of crossed guide directional couplers for feeding an electronically scanned antenna array is known in the art, being described for example in copending U.S. application Ser. No. 621,001 filed Mar. 6, 1967, by J. A. Algeo, et al., and owned by North American Rockwell Corporation, assignee of the subject invention. The use of such coupled crossed-guide arrangement in microwave antenna arrays provides an extremely compact and efficient array, as shown for example in copending U.S. application Ser. No. 699,756, filed Jan. 15, 1968, by C. A. Wiley, assignor to North American Rockwell Corporation, assignee of the subject invention.

A desired property of such couplers is constant coupling as a function of energy wavelength or microwave frequency, for utilization in frequency-scanned type electronically scanned arrays or in frequency diversity type electronic counter-measure applications. In other words, wideband response is desired. In this way, the aperture amplitude distribution of the array and the load power required by the array are unaffected, whereby variations in antenna sidelobe level and gain performance with frequency may be avoided.

Although crossed guide couplers are mechanically a most compact form of coupler, the common wall thereby provided presents a restricted coupling area. In the prior art, multihole coupling arrays in such crossed guide couplers have not provided tight coupling independent of frequency. The factors affecting the extent of such variations in coupling are: the frequency bandwidth of interest, the degree of coupling sought, the ratio of the design center frequency (of the bandwidth of interest) to the wave guide center frequency, and the type of coupling aperture employed.

For example, the wider the bandwidth of interest, the greater the variation in coupler performance over such bandwidth. Also, as the size of the aperture is increased, tighter coupling is provided; however, such increase in aperture size also causes the aperture to approach a resonant condition which further aggravates the frequency dependence of the aperture coupling performance. In other words, the performance combination of tightness of coupling with broadband response tends to be anomalous.

3,537,037 Patented Oct. 27, 1970 Further, the ratio of the center frequency of the design bandwidth to the waveguide center frequency is significant because the aperture will tend to resonate at high frequencies, while at low frequencies the rapid change of guide wavelength with respect to frequency results in coupling variations as a function of frequency.

The prior art of broadwall-to-broadwall crossed guide couplers has employed, alternatively, circularapertures and crossed slots as coupling apertures. Such circular apertures present the advantage of low coupling efficiency and poor directivity. Crossed slot apertures, on the other hand, are more expensive in manufacture, but allow moderate coupling with good directivity. Neither of these coupling apertures provide constant coupling over a broad bandwidth (i.e., a brandwidth approaching that of the associated waveguides of a crossed guide pair).

The utilization of crossed slot apertures in a crossed guide coupler is taught, for example, in U.S. Pat. 2,667,- 620 to Riblet, which reference also teaches the use of a single slot aperture oriented parallel to the longitudinal axis of one guide of a crossed guide pair for the coupling of a non-directional energy mode. In fact, Riblet re quires two feed guides in cooperation with his output guide, and employs crossed slot apertures, the longitudinal axes of which are-parallel to the walls of his crossed guides. The use of pairs of mutually parallel slots oriented parallel to the longitudinal axis of one guide of a crossed guide pair, for directional coupling, is shown in copending U.S. application Ser. No. 649,166 filed June 27, 1967, for a Crossed Guide Directional Coupler by J. A. Algeo and owned by North American Rockwell Corporation, owner of the subject invention. However, the device of Algeo requires two feedguides in cooperation with an output guide to effect selected directivity of a preselected polarization. A discussion of the prior art in microwave directional couplers is included in the section entitled Directional Couplers Section, at page 147, et seq., in vol. I of Advances in Microwaves, edited by Leo Young, Academic Press, Inc. (1966). Such reference includes a description of a Riblet T type coupler, pluralities of which are used in combination to effect increased coupling between two mutually parallel guides, and not to effect directional coupling between a pair of mutually crossed guides. Also, the component head and arm slots comprising such T are respectively perpendicular and parallel to the sides of the mutually parallel rectangular guides.

SUMMARY OF THE INVENTION By means of the concept of the subject invention, the above described limitations of the prior art are avoided and a directional coupler for a crossed pair of rectangular waveguides is provided which demonstrates wideband response with good coupling and which is convenient and inexpensive to manufacture. The common wall of the crossed guide pair is apertured, a first and second mutually spaced aperture thereof being narrow slot shaped, each slot being parallel to a mutually exclusive one of the two diagonals of the common wall.

Accordingly, it is an object of the invention to provide an improved crossed guide directional coupler.

It is another object of the invention to provide a crossed guide directional coupler having wideband response.

It is a further object to provide a wideband crossed guide directional coupler which is convenient and inexpensive to manufacture.

These and further objects of the invention will become apparent from the following description, taken together with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, partially torn away, of a cross-guide pair embodying the inventive concept;

FIGS. 2A and 2B are plan views of the common wall of the device of FIG. 1, illustrating the geometry of an exemplary arrangement;

FIG. 3 is a perspective view, partially torn away, of an alternate embodiment of the inventive concept; and

FIG. 4 is a plan view of the common wall of the device of FIG. 3, illustrating the geometry of an exemplary arrangement.

In the figures, like reference characters refer to like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is illustrated a perspective view, partially torn away, of a directionally-coupled crossed pair of rectangular waveguides 10 and 11. Guides 10 and 11 are broadwall-to-broadwall coupled by means of an apertured common wall 12 having a first and second elongated coupling slot or narrow slot-shaped apertures 13 and 14, which apertures are mutually spaced and respectively oriented parallel to a respective one of the diagonals 15 and 16 of a common wall 12. More particularly, the shape of apertures 13 and 14 is a like elongated slot having a centroid oppositely disposed about the intersection of diagonals 15 and 16 and lying upon a common one 15 of such diagonals. Where the broad wall dimensions of the crossed guide pair are like, then the shape of the common wall 12 is square and the two diagonals 15 and 16 are oriented at 45 to the longitudinal axes and sides of the crossed guide pair. Also, a common transverse dimension, d, is maintained between the centroids of apertures 13 and 14, as measure across the broadwall dimension of either of guides 10 and 11, as shown more particularly in FIGS. 2A and 2B. Such center-to-center spacing, d, is selected from considerations of the bandwidth center of the bandwidth of interest.

The common longitudinal dimension, 2L, of each apertures 13 and 14 is selected from considerations of the degree of coupling desired, maximum coupling being provided by a slot dimension 2L as large as may be accommodated by the location of the centroid of aperture 13 within the confines of common wall 12. Although the apertures in FIGS. 1 and 2 are illustrated as narrow slots having parallel sides, elliptically shaped slots of near-unity eccentricity may be employed.

In a selected application for an antenna feed system which is operated over a 20% frequency band (e.g., a bandwidth equal to 20% of the center frequency) and where the low frequency limit thereof is 10% above the waveguide cut-off frequency, it has been found that an optimum value of the dimension d is .644a, where a is the broadwall width. Further, a 3.5 db improvement in coupling may be obtained by reducing the height b' of the primary (input) waveguide to .447b, where b is the height or narrow wall dimension of the input waveguide.

In normal operation of the arrangement of FIGS. 1 and 2, energy applied at an input port D of primary guide 11 is coupled to port A of secondary guide 10. Also, ports B and C are intercoupled. In other words, that pair of ports are intercoupled which are closest to a given common aperture. Those mutually contiguous ports are intercoupled which are also contiguous with the diagonal on which apertures are located. Nominally, in waveguide design the narrow wall dimension, b, is selected as being one-half the broadwall dimension (b= /2a). Now, the coupled power by means of the arrangement of FIGS. 1 and 2 can be demonstrated analytically as being inversely propositional to the square of the waveguide height. Accordingly, a 6 db increase in coupled power may be obtained if both the primary and secondary guides 10 and 11 are reduced from full height to half height mm g The directive coupling properties of the arrangement of FIGS. 1 and 2 may be demonstrated and estimated by the following analytical treatment, employing the method described in the article Theory of Diffraction by Small Holes by H. A. Bethe, at pages 163-182 in volume 66, 1944 of Physical Review. Bethes equations for the forward coupled wave A and the backward coupled wave B, assuming a wave of unit amplitude incident at one port of two rectangular waveguides are given as follows:

where )\g is the guide wavelength; a and b are the waveguide broad and narrow dimensions as shown in FIGS. 1, 2A and 2B which depict the Waveguide orientation, slot orientation and coordinate system; M and M are the magnetic polarizability of the apertures in the uand v-directions, and P is the electric polarizability; H is the magnitude of the u-component of the magnetic vector in the primary guide that would exist in the center of the aperture if it were replaced by a conducting wall, while H is the magnitude of the u-component of the magnetic vector in the secondary or excited guide and is also evaluated at the center of the aperture; E is the electric field strength in the primary guide. Corresponding definitions apply to the v-components of the electric and magnetic fields. The superscripts and refer to the normal modes propagating in the positive and negative directions, respectively. Thus, H is the magnitude of the v-component of the magnetic vector when a wave propagates in the positive direction of the primary guide. For identical guides, the value of the field components of the H mode are given by:

when a unit amplitude wave travels in the positive zdlrection. For a wave traveling in the negative z-direction, the field expressions become:

where 5 is the propagation constant equal to 21r/ \g. Following common usage, the time dependency factor e has been omitted from the field component expression.

The apertures shown in FIG. 2b consist of narrow slots with parallel sides. However, they may be considered to be elliptical slots of near unity eccentricity with major axis 2L and minor axis 2W. With this assumption, the magnetic and electric polarizabilities of slot 13 become (See the article Directive Couplers by N. Surdin at pp. 725-736 of vol. 93, Part IIIA, 1946, Journal of IEE). The polarizabilities of slot 2 are For very narrow slots, the transverse magnetic and electric polarizabilities may be neglected. Also, assuming that there is no interaction between slots, the total for- 'ward and backward coupled waves may be expressed as A=A1+A2 B=B +B (1 where A and B are the respective forward and backward coupled waves from slot 1 and A B are the corresponding coupled waves of slot 14. Noting that M of slot 1 is equal to M of slot 14 and setting the electric and transverse magnetic polarizabilities to zero, the total forward and backward coupled wave expressions become where M =M =M We now evaluate the fields coupled into the secondary waveguide when a unit amplitude wave propagates in the positive direction of the primary guide. At slot 13 2:0,

Hence H =H sin +H, cos 0 a: .19 am: H L

,y sin a sin 9-H 2a cos a cos 0 and for slot 14 located at z=d,

a-i-d (II-T H,, =H sin 0+H, cos 6 E --m 21 I! -m H sin a sin 0e +1 2a cos a cos 00 The slot coordinates in the secondary waveguide are X, Y and Z, and the position of slot 13 is Hence H =H cos 0+HZ sin 0 Substituting the appropriate field expression H cos 2a cos 0+3 2a sin 2a sin 0 (19) and Ti 'ZQ H" cos 2 cos 0+3 2a sin 2a sin 0 (2o) Slot 14 is located at a 01 Z d, X

20 Thus H H cos 0+Hz sin 0 E 'M- L; )fld H (cos 2a cos 0+ 2a sin 2a sin 0 e' d Ag 7rd L flu H cos 2a cos 0+ 2a sin 2a sin 0 e 1 The product H H yields M12 7rd 1rd 2 2 H H +sin 0 cos 0 sin +cos 2a .Ag 1rd 1rd 2 2 g sin 2a cos (sin 6 cos 0) In the same manner we find that i d d 2L 2L pu m sin 0 cos 0 a S1I1 -l-cos 2a n 1 2 ii 2 2 J 2 sin 2a cos 2a (sin 0 cos 6) (24) From inspection it is seen that the sum of Equations 23 and 24 is zero. Accordingly, the substitution of Equations 23 and 24 in Equation 16 results in a zero value therefor. Since the value of Equation 16 is zero, the backward coupled B wave is zero and independent of 0 and frequency.

The total field propagating in the positive Z-direction is found to be 21rM i fi X92 7rd 1rd A- ab M sin 20 sin M sin 2a cos 2a 2 sin sin j sin 20 sin -cos sin sin 7L4 sin lid 2a Ag 4a a Ag and the power in the forward direction of the excited guide is proportional to n 4 12 i 2 1 a [A] azbz sin Ag s1n M M2 sin 20 2 2 zsin g g-cos? sin Substituting for M we obtain We note from Equation 27 that the coupled power is inversely proportional to the square of the waveguide height. Therefore, approximately a 6 db increase in coupled power can be obtained from a given aperture if both the primary and secondary guides are reduced from full to half height. Due to the assumptions made in the derivation, the results obtainable are only approximate; however, they are more than adequate for an initial determination of a particular coupler design, and adjustments can be readily made to bring the coupling value within the desired value over the bandwidth of interest.

The arrangement of FIGS. 1 and 2 provides almost constant coupling and good directivity over a wide bandwidth. However, it has been found that tighter coupling may be provided, if required, by the alternate arrangement of FIG. 3.

Referring to FIG. 3, there is illustrated a perspective view, partially torn away, of an alternate embodiment of the inventive concept, in which the two elongated slots are mutually contiguous, forming a composite T- shaped aperture 34. In other words, the centroid of each of the mutually contiguous slots 13 and 14 comprising aperture 34 lies upon a common one of the common-wall diagonals and on a common side of the intersection of the two diagonals. The length of the slots defining the T aperture 34 (dimensions A and B in FIG. 4) are selected to minimize the coupling variation in the design bandwidth (bandwidth of interest). For example, primary guide 11 is designed as a half-height guide relative to secondary guide to effect increased coupling. However, the stronger the coupling the poorer the impedance match within the guide. A second T-shaped aperture 35 is provided in the arrangement of FIG. 3 for improving the degree of directivity obtained. Such second aperture is oriented at 90 to first T 34 and situated on a second one of the common wall diagonals and 16, the length of the slots defining T aperture (dimensions C and D in FIG. 4) being selected empirically.

An inductive post 18 and 19 is employed in each of the guides 10 and 11 of the crossed guide to reduce impedance mismatches caused by the coupling aperture, the respective location of a respective one of inductive posts 18 and 19 in an associated one of guides 10 and 11 being a function of the guide height. For an intended application within the bandwidth defined by a lower frequency of 2.86 gigahertz and an upper frequency of 3.46 gigahertz the T arrangement of FIG. 3 has employed the dimensions indicated in FIGS. 3 and 4.

In normal operation of the arrangement of FIGS. 3 and 4, ports D and A are intercoupled and ports C and B are intercoupled. In other words, those mutually contiguous ports are coupled which are also contiguous which that diagonal on which the ancillary T-slot 35 is located.

Accordingly, improved directional coupler means for a pair of crossed microwave waveguides has been disclosed.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. A microwave energy coupler for a crossed rectangular guide pair and comprising an apertured common rectangular broadwall of said crossed guide pair having a first and second mutually spaced T-shaped aperture, each said aperture being two mutually contiguous elongated slots, forming a composite tee, each of said mutually contiguous elongated slots being respectively parallel to a respective one of the two diagonals of said common wall. -l

2. A microwave energy coupler for a crossed rectangular guide pair and comprising an apertured common rectangular broadwall of said cross guide pair having a first and second mutually spaced T-shaped aperture, each said aperture being two elongated slots the centroid of each slot of said aperture lying upon a common one of said two diagonals on a common side of the intersection of said two diagonals, said slots of said aperture being mutually contiguous, each of said slots forming said T-shaped apertures being respectively parallel to a respective one of the two diagonals of said common wall.

3. .A directional microwave energy coupler for a crossed rectangular guide pair, said coupler including an apertured common rectangular broadwall of said crossed guide pair and having a first and ancillary second mutually spaced slotted T-shaped aperture of said common wall,

said first aperture consisting of two mutually contiguous elongated slots forming a T -shape, the centroids of said slots lying upon a common one of the diagonals of said common wall and on a common side of the intersection of said two diagonals, and said slots being respectively parallel to a respective one of said two diagonals of the said common Wall;

said second T-shaped aperture oriented at to said first aperture and situated on a second one of said diagonals.

4. A directional microwave energy coupler for a crossed rectangular guide pair, at least one guide of said guide pair comprising a half height guide, said coupler including an apertured common rectangular broadwall of said crossed guide pair and having a first and ancillary second mutually spaced slotted aperture of said common wall,

each of said first and second apertures consisting of two mutually contiguous elongated slots forming a T-shape, the centroids of said slots of one aperture lying upon a common one of the diagonals of said common wall and on a common side of the intersection of said two diagonals, and said slots being respectively parallel to a respective one of said two diagonals of said common wall; said second T-shaped aperture oriented at 90 to said first aperture and situated on a second one of said diagonals; and impedance matching means comprising an inductive post inserted in each of said crossed guides.

References Cited UNITED STATES PATENTS 2,602,859 7/1952 Moreno 333-10 2,870,4-19 1/1959 Riblet 333-10 3,230,483 1/1966 Kinsey 333-10 3,377,571 4/1968 Kinsey 33310 RICHARD A. FARLEY, Primary Examiner D. C. KAUFMAN, Assistant Examiner US. Cl. X.R. 

